hybrid lighting

Christ en Jacqueline Monden of De Kruidenaer in Etten-Leur had hybrid lighting installed in their new high-tech greenhouse, in which they grow basil, just over a year ago. Until now, their experiences have only been positive: a higher production yield, better quality, a longer lifespan and improved plant resilience.

Last year, Christ Monden had a new 14,000 m2 greenhouse built with diffuse glazing, a humidifier and hybrid lighting for the cultivation of basil on water (hydroponics). The basil is grown in six 2,100 m2 containers, which Christ refers to as ‘ponds’. The young plants are started in this greenhouse as well. They are transplanted to ‘floaters’ with plant holes after 1 to 2 weeks. The ponds are located parallel to one another, without any aisles in between. As a result, optimum use is made of the available space.

Short cultivation cycle

On one side of the greenhouse, the floaters with the young plants are set to drift in the water and – after 3½ to 5 weeks – they are lifted out at the other side of the greenhouse for harvesting. This is a short cultivation cycle, thanks to the intensive lighting, which ensures a constant supply of fresh basil six days a week. In addition to basil, the Brabant-based nursery also grows 23 other types of herbs, lettuce (grown hydroponically) and sweet peppers (amounting to a total of 8.5 hectares of greenhouse cultivation and 23 hectares of open-field cultivation with tunnels) for the retail, food service and export industries.

Basil likes warmth, but not too much. In this case, LED lighting is the ideal solution.

Chequerboard pattern

In addition to diffuse daylight, the plants also receive artificial light provided by Oreon Grow Light (260 KVA) LED fixtures, in combination with Gavita SON-T (460 KVA) lighting, with a maximum output of 100 μmol/s (the output of the LED system alone is 50 μmol/s). The light fixtures are suspended above the crop in a chequerboard pattern. Their design (lighting plan) and the installation of the system were carried out by Voshol Warmte-Elektrotechniek. Oreon LED fixtures have been installed, among others, at lettuce grower Boer den Hoedt and Koppert Cress.

Water cooling

One of the unique features of the Oreon LED fixtures is its water cooling system. This has various advantages according to Jos Duijvesteijn of Voshol. ‘Basil likes warmth, but not too much. In this case, LED lighting is the ideal solution LED is composed of diodes, which get very hot at the back. Heat is, of course, detrimental to the useful life of electronic devices. Thanks to the cooling system, we can keep the temperature of the lamp between 35 and 45 degrees Celsius, thus substantially extending their useful life (L90 = 35,000 hours, ed.). The heat discharged through this process (50 degrees Celsius) is reused by De Kruidenaer – for heating the ponds, for example. This means that no energy is lost.’

The heat discharged through this process is reused – for heating the ponds, for example.

Test set-up

Christ explains why he chose to install the Oreon LED fixtures: ‘They came out best in a test that was conducted prior to the construction of the greenhouse. The plants grew very uniformly and were super strong. In the end, we opted for 50% SON-T and 50% LED because basil can really use this extra heat. We can light the plants exclusively with SON-T, or exclusively with LED, or with both types of lighting. Thanks to the LED fixtures we can give the plants light for much longer, both in the morning and in the evening, particularly at this time of the year.’

Prolonged cultivation

The lighting is switched on as frequently as possible, up to a maximum of 20 hours per day. ‘Last year, I used more SON-T lighting than I had expected. The crop was able to tolerate the extra heat very well. We use SON-T exclusively or in combination with LED, but have not used exclusively LED all that often yet.’ Christ thinks that the LED fixtures were switched on for 2,000 hours last year. Thanks to the LEDs he can give his plants prolonged light and start earlier in the season – four to six weeks each season, he estimates.

We were able to produce the anticipated volumes, and the quality is better. The plants also have a longer lifespan.

Longer lifespan

The result after growing plants under hybrid lighting for one year is positive. We were able to produce the anticipated volumes, and the quality is better, says Christ. Additionally, judging from customer response, Christ noted that the plants have a longer lifespan. He also has to purchase less product in addition to what he grows in order to meet customer demand. ‘They prefer basil grown in the Netherlands, with a PlanetProof Hallmark (previously Milieukeur). We were able to get through last summer – with temperatures above 35 degrees Celsius – much better than I had anticipated.’

Resilience

Christ is convinced that the LED lighting contributes to improved plant resilience: ‘During the tests, we noted stronger plans, with sturdier leaves and shorter internodes. The colour was better, too. I am sure that this gives the basil has greater vigour. These results are also reflected in our crops. Basil is a highly demanding crop, a real “prima donna”. Things can be completely different from one day to the next.’

I am convinced that the LED lighting contributes to improved plant resilience.

Investment

Although the investment in a new greenhouse ended up being much greater due to the purchase of the LED lighting fixtures, Christ and Jacqueline Monden are very satisfied about the results. ‘We grow almost all the basil ourselves, particularly in the season. What I am most satisfied about is the quality of the crop. We aim to grow the best basil in Europe. In about five years’ time we want to stop growing sweet peppers entirely. It has been going really well until now,’ he says with a wink that shows his marvellous feeling for understatement.

More output

For which types of growers is hybrid lighting interesting? ‘The product range is becoming increasingly broad,’ says Jos Duijvesteijn of Voshol. ‘There are plenty of possibilities in the cultivation of herbs, particularly if heat is a problem. The same goes for growing lettuce. Hybrid lighting is really on the rise among tomato growers. We are currently rolling out the first projects, with LED top lighting in combination with SON-T. A gigantic advantage is that the LEDs will give you a much greater output, with a smaller capacity. This means that you can get more micromoles from the same capacity.

Light patterns

We are very enthusiastic about Oreon’s water-cooled fixtures. This product is stable, and has pretty much be reached the highest level in terms of engineering. Also, once it has been installed there is nothing you need to do. You can also generate a diversity of light patterns with LEDs, for the vegetative and generative growth of the crop. The latter still requires a lot of research, but I can certainly identify opportunities here.’

A trial with hybrid lighting (SON-T + LED) at Dutch tomato nursery Gebroeders Koot has yielded good results. The LED lamp used in the trial, which was developed on British soil with Dutch input, offers several advantages. One stand-out benefit is its clever design which makes it easy to integrate into existing SON-T installations.

Yields up by more than nine percent after seven months (weeks 48-26). That was the auspicious outcome of a greenhouse trial at Prominent growers Gebroeders Koot in Poeldijk, the Netherlands, where a tomato crop grown under 150 μmol/m2/sec SON-T grow light was compared with an identical crop supplemented with 58 μmol deep red with a little blue LED light. Geert Koot, who had had no previous experience in growing under grow light, was very impressed. “I hadn’t expected the higher light level to make such a difference,” he says. “That will appeal to a lot of growers. The same goes for the lamp itself, which has a surprisingly simple design. It’s fully interchangeable with SON-T, so it fits seamlessly into an existing system.”
“A lot of thought has gone into the functional design,” cultivation specialist Maarten Klein adds. He and his assistant, Tim Valstar, oversaw the trial, which was run on behalf of the British LED manufacturer Plessey. Klein, who has had a lot of experience with grow light, developed this lamp in collaboration with the technology company.

Smarter design

“Most LED systems are difficult if not impossible to integrate into existing lighting installations,” Klein continues. “Growers looking to switch to hybrid lighting currently have to install a whole new system alongside their existing one, often with extra C profiles. That pushes up the cost and results in more light interception, which causes problems all year round. Plessey Semiconductors in Plymouth wanted to eliminate these problems.”
To test the practical value of the lamp in the greenhouse setting, Klein approached several Dutch nurseries. In addition to Gebroeders Koot, trial setups were installed at nearby alstroemeria and gerbera growers and a pot plant nursery.

Trial setup

Although Gebroeders Koot were not growing tomatoes under artificial lighting, they did have a SON-T system in place in a section that had previously been let to another grower. These 1000W lamps supplied 151 μmol/m2/s extra grow light and, of course, the usual radiated heat. LED lamps were added in one bay, ramping up the artificial light level to 209 μmol.
Tim Valstar assisted with the trial and, together with Geert Koot, took measurements in the trial and reference sections. All the relevant crop and fruit features of the variety grown, Brioso, were recorded, varying from growth rate and stem thickness to leaf size, leaf colour, fruit weight and Brix value.

Results

The plants arrived in the greenhouse in week 46. “That’s later than the usual for an artificially lit Brioso crop – they would usually go in in mid-October – but the lighting period was long enough to get a reliable impression of any differences,” Koot says. “The plants developed well in both light environments. But the plants under the higher light level were that little bit stronger with slightly thicker stems and more dark green leaves.”
Due to the extra vigour, the plants under the hybrid lighting regime held the first trusses for longer and they were harvested a few days later than those in the reference sections. The higher yield potential quickly expressed itself in a higher average fruit weight. To maintain the desired fineness, one fruit more was kept on the truss (11 instead of 10) from the tenth truss onwards, without the plants forfeiting vigour.
Valstar: “After week 26 we stopped taking measurements and were able to take stock.” The harvest under the hybrid lighting regime was 38.32 kg per m2 compared with 35.04 kg under SON-T. That represents an increase in yield of 9.35%. The average fruit weight was also slightly higher than under SON-T, at 39.2 grams compared with 38.8 grams.

Flexible use

The attractive increase in yield can’t be ascribed solely to the higher light levels in the periods when both systems were in use. The SON-T system was switched off and the CHP unit shut down for maintenance at the beginning of week 19, whereas the LED system was used from 4 am to 7 am for a further three weeks.
“The option to only use the LED lamps either end of the lighting season would be an extra benefit,” Klein says. “Those are often the times when you don’t need the radiated heat produced by the SON-T lamps. LEDs have virtually no impact on the climate. You can always switch them on if you need more grow light. And because they are much more energy-efficient than SON-T lamps, you also have more flexibility when it comes to deciding whether to generate the energy yourself with CHP.”

375 and 600W

Klein is keen to point out that the prototype trialled at Gebroeders Koot was developed exclusively for research purposes. But the lamp has since undergone further development and a commercial 375W version was launched at IPM 2017. All the LEDs are now in one bay and the fitting, which has integrated cooling ribs, can be attached directly to the trellis.
The lamp is called Hyperion 1000 because it has a photon flux of 1000 μmol/s. “Because of the higher uptake of deep red light, it’s the equivalent of a 600W SON-T lamp but it uses 40 percent less electricity,” the cultivation specialist says. “The producer has also recently brought out a more powerful 600W version which is the equivalent of a 1000W SON-T lamp.”

Ten years ago

There is a lot of added value in the new lamp, Koot believes. “It’s efficient, it has a broad spectrum, and its clever design makes it easy to incorporate into an existing system. That will appeal to a lot of growers. I’m also quite impressed. But because of my age and the fact that I have no successor in place, I have decided not to invest in any more grow lights now. If this trial had taken place ten years ago, I would almost certainly have gone for them. But we very much enjoyed taking part in the trial.”

Summary

A new type of LED lamp produced in the UK is achieving interesting results. The clever design makes the lamp particularly attractive. It can be attached to the trellis without the use of C profiles and can be integrated into existing 600W SON-T systems with standard connectors. A more powerful version equivalent to a 1000W SON-T lamp was brought out earlier this year.

Hoogenboom Alstroemeria in Nieuwe Wetering in the Netherlands expanded its grow light system last year. In one section of the greenhouse they have installed LED lights in between the rows of controllable SON-T lights, ramping up the light output there by 50% to 150 μmol/m2/s. “You don’t always need the radiated heat that SON-T lights produce. We can light our crops more intensively and more flexibly with this hybrid system,” partner Dick Hoogenboom explains. “It produces visible results.”

Brothers André and Dick Hoogenboom have been growing alstroemeria in their 2.9 hectare greenhouse in Nieuwe Wetering since 1985. Their aim is to produce the most beautiful, strongest and healthiest cut flowers. “It takes a long time to establish a good reputation, but once you have it, you can lose it again very quickly,” grower Dick says. “We have to work hard to stay ahead of the field because the competition never stands still.”
Grow light plays a key role in the battle for quality. The brothers installed their first lighting system in the late 1990s, with 600 W SON-T lights producing 4000 lux or 50 μmol/m2/s of PAR light.
“We increased the output every time we replaced or renovated something,” the grower explains. “We have had around 100 μmol for eight years now. Five years ago we upgraded our entire power system and we replaced the 600 W bulbs with 1000 W ones, which we can either have running on full or half power.”

SON-T or LED

The growers wanted to up their lighting capacity yet again last year. First they had to decide whether to go for extra SON-T lights or install a second, LED-based system.
Hoogenboom: “The easiest and cheapest option would have been to put in extra SON-T lights. But we didn’t need even more radiated heat so we opted for the more expensive LED system. We deliberately went for Phillips lights; they weren’t the cheapest but we have the most confidence in the quality, reliability and after-sales service you get from this supplier.”

Lighting strategy

LED lights don’t radiate heat onto the crop and can be used whenever additional grow light is needed. Hoogenboom aims for a day length of 16 hours. Between 1 August and 1 April the daylight is always extended with artificial light (and radiated heat) from the SON-T system. At the beginning and end of this period the days are usually long and light enough for them to manage with a light intensity of 50 μmol.
“We switch the lights on full power on around 1 October,” the grower adds. “Now that we also have LEDs in the greenhouse, I can decide which system to switch on first. If the temperature is still warm enough, we will use the LED system. I have often used the LED lights at times when the lighting in the section that only has SON-T was left switched off because it would otherwise have got too hot there. You can get rid of excess heat by opening the vents, of course, but that costs you CO2 as well, which impacts on growth.”
During the winter and on chilly spring days, the grower switches the SON-T system on first because the radiated heat is very useful. At that time of year the LEDs are added in to ramp up the light level to 150 μmol when necessary.

Noticeably higher yields

The grower is convinced that there is scope to get even more out of his hybrid lighting system by working with crop adviser Marco de Groot and tweaking the climate. Although the new LED system has only been in use since November last year and at the time of writing he has yet to experience using it between 1 August and 1 November, he is happy with the results so far.
“Under the higher light level we used from 1 November onwards, the crop has been producing noticeably thicker, firmer and stronger stems,” Hoogenboom says. “What’s more, yields are quite a lot higher. Depending on the variety, by 1 April we were cutting between 30 and 50 stems more per square metre than usual.”

More watering

Hoogenboom also noticed less bud dehydration and deformation in the greenhouse with the higher light levels. On the other hand, he has noticed that more light means more watering. “The crop is more active so it transpires more. I didn’t really take that on board to begin with. But there was a leaking tap in one bay so it was wetter there than in the other irrigation sections for some time, and in the spring we noticed that that crop was fuller and was producing more flower spikes. So I’m convinced there is even more room for improvement. That makes us even more confident of a positive return on our investment.”

Summary

Alstroemeria can get off to a good start with higher light levels than are generally used at present. SON-T lights also emit radiated heat and meet the plant’s basic needs. If you don’t need extra heat at the top of the plant, LED lighting can boost light levels in the winter months, which translates into more and stronger flower spikes and less bud dehydration and deformation.

“You still need to do the maths”

Crop adviser Marco de Groot is keeping a close eye on the developments with grow light in alstroemeria. He helped the Hoogenboom brothers do the maths to underpin their investment decisions. “Lighting levels in many ornamental crops continue to rise,” he says. “Alstroemeria is no exception. And that has to happen if our growers are to compete with Africa, where alstroemeria cultivation is expanding rapidly. More light is good for the quality of the crop, yields and more regular workforce deployment between October and June. But you do need to take a critical look at where you’re coming from, where you want to go and the best way to get there.”

The adviser continues: “For growers currently working with low light levels, a heavier-duty SON-T system would be the best choice. If you are already using slightly more light and don’t need extra heat, you should rather opt for LED. Don’t forget that unlike rose and gerbera, photosynthesis in alstroemeria has a response time of around 30 minutes. LED light doesn’t increase the leaf temperature, unlike SON-T light. If the leaf temperature is too low, that can delay the response time even more. Extra LED light is only effective when the basic level is covered by SON-T light.”
In addition, the light response varies from variety to variety because of the strong genetic variations. Virginia can use extra light efficiently whereas other varieties often respond more slowly. “It’s important to take these aspects into account,” De Groot concludes.

More growers gaining experience

According to plant specialist Stefan Hendriks of Philips, a good number of ornamental growers are gaining experience with hybrid lighting or 100% LED. “We are seeing a growing trend towards LED in the market as growers become more and more aware of the advantages. But it is important to bear in mind that it’s not simply a question of installing LEDs and carrying on as usual. You also have to control the climate differently. LED lighting provides you with an extra tool because you can control heat and light separately, which you can’t do with SON-T.” In addition to research in vegetable crops, Phillips is also working with ornamental crops such as rose, gerbera, alstroemeria, chrysanthemum and phalaenopsis.
More and more growers are realising that LEDs deliver benefits for the crop, quite apart from the fact that you can’t always increase light levels with SON-T in some crops because of the excess heat they produce, Hendriks says. In practice, you can get more lighting hours out of LED than with the same crop under SON-T. “This boosts yields and quality. The use of LED is growing, and with increasing success. This trend looks set to continue and we expect that LED systems will most likely take over from SON-T completely in the longer term.”

Completely separately from one another, two Belgian rose growers with premises barely ten kilometres apart came to the conclusion that a combination of SON-T and LED top lighting is the best solution for their nurseries. This makes them the first rose growers to use hybrid top lighting. Since then, other growers from as far away as Russia have come to take a look.

Electricity is expensive in Belgium: it costs around € 35-40/MW more than in the Netherlands as a result of higher transmission and distribution prices. So it makes sense to generate as much as you can yourself and buy in as little as possible. This was a key factor in growers Rozen Scheers’ and Wimceco’s decisions to install combination lighting.
Both nurseries were looking to upgrade their partly outdated systems and were thinking about how to do so without having to start buying in a whole lot more electricity. At Scheers they were able to increase the light level from 100 to 150 µmol/m2/s using exactly the same amount of power. Wimceco went from 130 to 180 µmol/m2/s with a modest additional input of 150 kW.

Practical experience

Wim and Annelies Scheers’ nursery Rozen Scheers in Kontich in the province of Antwerp has 3.8 hectares of glass. They have 15 varieties in their product range: popular roses such as Avalanche and Red Naomi along with special varieties like Jumilia and Cupcake. Sales take place via Euroveiling in Brussels, but direct sales to consumers are also an important channel.
Wimceco in Boechout, also in Antwerp province, grows Avalanche and Sweet Avalanche on 1.75 hectares. They also grow a few lesser varieties to vary sales to consumers. Their roses are auctioned at Euroveiling in Brussels and FloraHolland. Co-owner Danny van Nuffelen: “Our lamps and reflectors needed replacing as their output was dropping. We wanted to increase the light level but we didn’t want to buy in too much electricity.”
That could be done by combining SON-T (HID) with LEDs, but no growers had ever tried this combination with cut roses before. “That’s why our suppliers Agrolux advised us to go and visit Leo van der Harg, a pot rose grower in Vierpolders in the Netherlands. He has been using a hybrid system since 2014. His experience went some way to removing the uncertainties – after all, he is also a rose grower. What’s more, our starting point was that a photon is a photon, no matter what kind of lamp it comes from,” he explains.
At around the same time, Wim Steeghs of Philips Horticulture LED Solutions suggested setting up a trial with hybrid lighting at Rozen Scheers. That was when Wim Scheers first heard that his colleague ten kilometres down the road had similar plans. He also went to visit Van der Harg.

Climate control

A complete switch to LEDs only was not on the cards. “That really is too new and would be much more expensive. Also, you would have to completely revisit your climate control because you wouldn’t have the extra heat from the HID lights,” Van Nuffelen says. “And anyway, LED light on its own is too red to work in. No research has been done yet into what effect that has on people. It looks very dark.” He and his brother Bart decided on a 50/50 split between HID and LED. The lights hang in the same line: three LED fixtures followed by one 1,000 W SON-T.
Fellow grower Scheers uses LEDs for 70% of the light and SON-T for 30%. “We have two CHP units of 1.6 and 1.2 MW. We wanted more light but without using any more electricity. That’s why we decided on this combination. We have two continuous lines of LEDs in each bay interspersed with one HID reflector every metre.” The system is fitted in a 1.5 ha greenhouse which makes up 40% of their total greenhouse area. It was installed by Dutch B-E De Lier. In both cases, the LED fixtures are Philips GreenPower LED top lighting.

Subsidy

The availability of a subsidy from the Flemish Agricultural Investment Fund (VLIF) made the decision on the high investment costs a little easier. Nominally, the VLIF contribution for LED systems is 30% of the investment costs, but it works out at less than that in practice on account of special conditions such as a cap on the investment amount per square metre. The ultimate support percentage is around 20-25%.
The systems were installed in November 2015, so the growers now have well over a year’s experience with hybrid lighting under their belts. They are reckoning on a payback time of 4-5 years.

Comparison

The HID and LED lights at both nurseries can be switched on and off separately. Van Nuffelen: “This has made us more flexible. The LEDs are left on until there is around 250 W of daylight; the HID lights are switched off earlier. So the LEDs were on for 890 hours longer than the HIDs last year.” Fellow grower Scheers used his LEDs for 1,100 hours more than the HID lamps.
When switching to LEDs, a big question is whether or not the radiated heat from the HID reflectors needs to be compensated for. That can have advantages and disadvantages. In theory, the grower needs to ventilate less to get rid of surplus heat. This is less of an issue in Belgium than in the Netherlands because light emissions don’t yet have to be controlled with blackout screens, so Belgian growers are spared the situation in which the temperature under the closed screen rapidly gets too hot. Because the nurseries kept no records of how long the vents were open, it is difficult to comment on this. Moreover, the light level was higher in the new situation in both cases, making it difficult anyway to draw a comparison with the previous situation.

Heating up

On the other hand, the radiated heat can be a welcome way of preventing the head from cooling down compared with the rest of the crop. “We can use top heating to compensate for the heat from the HID reflectors. In theory the crop should cool down quite a bit in the new situation, but our measurements show that the temperature didn’t drop as much as we had anticipated,” Scheers says.
Advisor Steeghs has an explanation for this. “LEDs also provide some indirect heating. They are 55ºC and transfer their heat to the aluminium profile. That transmits the energy to the light and water in the greenhouse via longwave radiation. The crop isn’t warmed up directly but indirectly.”

Research

A trial with 100% LED top lighting above Red Naomi has been running at the Delphy Improvement Centre in the Dutch town of Bleiswijk since April 2016. The light level is 200 µmol/m2/s and OPAC heat exchangers keep the temperature at the right level. Steeghs: “Before the trial the growers had said that they needed the radiated heat from SON-T and that 100% LEDs would not produce good results. But so far the plants have been growing very well.”
The results achieved in Bleiswijk in the summer and autumn were surprising: heavy roses with a long stem, a consequence of the lower bud temperature due to the reduction in radiated heat combined with the cooling properties of the OPAC heat exchangers. Outside the trial, growers were having trouble getting good stem lengths in the warm autumn. It remains to be seen what the results of the trial in the winter will be.

Production

A year on from the investments, both growers are happy with their decisions. Van Nuffelen: “The system has definitely lived up to our expectations. We have an older crop but it is in better condition this year than last year and has survived the summer better. A big advantage of this system is that it makes you more flexible: you can go on lighting for longer on a hot day with LEDs without pumping in heat from the HIDs.” At the time of writing he was unable to say how much more production he was getting with the higher light level.
At Rozen Scheers the figure was 20%, with 30% for some varieties. “That is much as we expected, but it could be even more. We are trying to grow as organically as possible but we had a lot of trouble with thrips last year and we had to cut out unsaleable product. Once we have that under control, production will be up,” he says.
Convinced by the results, in early November he had exactly the same combination installed in another 1 ha greenhouse, again with a 70/30 split between LEDs and HID reflectors. This time the lights were supplied by Agrolux. The way the system is connected up means that his large CHP unit can power the LEDs throughout the whole nursery and the other unit can be switched off when only the LEDs are on.

Interest

Scheers plans to replace another two older low greenhouses in the future. As there is a stream running right behind the greenhouses, the authorities have made it a condition that he makes a contribution to solving the flooding problems in the area. His fellow grower in Boechout will be changing the old crop first. A new greenhouse may be added in the future.
In the meantime, there is a lot of interest from other growers: sometimes entire coachloads drop by. Philips regularly gets enquiries from Russian rose growers who want to see the impact of hybrid lighting on the crop in practice.

Summary

Belgian nurseries Rozen Scheers and Wimceco are the first cut rose growers to install hybrid top lighting. Their decisions were influenced by the high cost of electricity, subsidies and the ageing of their previous systems. Initial experience is good: production is up as a result of the higher light levels and the heat produced by the HPS lamps can be easily compensated for. Scheers has just installed the same system in a second greenhouse.

LED lighting allows energy consumption to be reduced in the cultivation of tomatoes with assimilation lighting. Even better: energy consumption can be cut in half. Eight PhD candidates and three post-graduate researchers are conducting research as part of the ‘Led it be 50%’ project to achieve this.

‘Switching from high-pressure sodium (SON-T) lamps to LED lighting will result in energy savings of 25% with regard to the conversion of electricity into light. In a few years that will even be 30%, since LEDs are becoming increasingly effective. A more even distribution of light across foliage by suspending LEDs at the right position dispersed throughout the crop will enhance light absorption by 15%.

‘Another variable is the application of different colours of light, which will enable you to control the intensity of the light throughout the day. This should lead to a photosynthesis intensification of 10%. We also want to investigate the possibility of sending relatively more assimilates to the fruit to enable 5% more fruit to be formed with the same photosynthesis level. A total amount of 60% in electricity can be saved on lighting.’

As LEDs produce little heat, will greenhouses require more heating?

‘Net energy savings of 50% are realistic. I don’t think that growers will need to raise their heating quotas, because my theory is based on the idea of crops being cultivated under higher humidity conditions. The humidity can be higher particularly during the night-time, so that less moisture will evaporate from the plants. Vaporisation costs energy, which is why we are seeking ways to cut back on vaporisation and to achieve cultivation under slightly more humid conditions.

‘A low evaporation rate and high humidity conditions allow you to save on heat. That has to compensate for the lack of heat otherwise produced by SON-T lamps.

‘Cultivation under higher humidity conditions, however, increases plants’ susceptibility to mildew and fungi. We hope to enhance the resistance in plants being grown under LED lighting through such measures as the controlled application of red light during the night.

‘We aim to achieve a production increase of 30% with the same amount of light - or the same production levels with 30% less light. But will professional growers opt for these possibilities?’ Marcelis has to smile. This question is reminding him of the introduction of a tomato variety 35 years ago. This variety could be grown at a lower temperature, but when exposed to normal temperature conditions the crop yielded decidedly more fruit. Growers unanimously preferred the latter option. ‘We examine the relationship between the amount of light used and the plant’s response to this. An entrepreneur will decide for himself where his priorities are.’

Is interlighting the answer to a more efficient use of light?

‘Light needs to be absorbed by a plant in order for it to contribute to its growth. Of all the light that shines on a plant from above, 5 to 7% is refracted. This is what you see when you fly over a greenhouse at night with the lighting on. It is not true that a portion of the light is refracted upwards on its own accord. The lamps direct their beams downwards. Another portion of the light is lost because it hits the ground. This is around 5 to 10%. In conclusion, another small portion of light is lost through transmission. This is the light that shines straight through a leaf.

‘The challenge lies in being able to reduce light loss, and to distribute light as evenly as possible. When placed directly beneath the lamps, a plant may receive an excess of light, and placed lower down, it may receive too little. In this case it’s better to consider not only vertical but also horizontal distribution. Interlighting, however, doesn’t solve this problem entirely, but it can cut back this loss considerably. You lose less light to the open sky and the ground.’

Interlighting doesn’t enable light to be projected at a big distance.

‘There is not a lot of light behind a leaf. There must be a way to improve that. Perhaps distribution could be improved with a different shape of leaf. Or you could reduce the size of the lead and experiment with adding colours to the light. I’m certain that much more can be achieved, but this will require a great deal more research.

‘SON-t lamps do not emit their light in a uniformly distributed manner across the crop; most of the light is absorbed by the topmost leaves. With a diffuse distribution improvements of 5% could be achieved.

‘Seventy per cent of all assimilates are absorbed into the fruit. This means that 30% remain inside the plant, but does the plant need this much? Suppose that you can get 75% to the plant through more efficient light control. This is an interesting aspect to take consideration.

‘Placing a diffuse sheet of glass under an SON-T lamp will take away too much light. And even if you make that light diffuse, the reflection remains and you still have less light at the bottom. The question for the industry is: this is what we can do with the sun, now what can you do with the lamp? There are still numerous possibilities with LEDS by placing lenses in front of the light source.

‘Five years from now growers will probably be using a combination of SON-T on top and interlighting in between the crop. But in the end, they will be using LEDs exclusively. I’m not clairvoyant. Perhaps SON-T lighting will make giant strides forward, but there are more development possibilities for LEDs.’

Do plants derive other substances when exposed to LED lighting in comparison to SON-T light?

‘LED lighting directed at the bunch in tomato plants will double the Vitamin C content. This immediately raises new questions for further research: how does that work? What colour light would you need to achieve this? Research on this is currently in full sway. Perhaps this will show us that we can increase other beneficial substances as well. It is doubtful that professional growers will soon be positioning their lighting directly around every bunch of tomatoes, but we do want to discover the principle behind this. Perhaps this will offer growers new possibilities. Everyone can grow tomatoes under diffuse glazing, but if you can grow tomatoes that have a beneficial effect on health, you can distinguish yourself on the market. Specific types of LED lighting could also increase these substances in other crops, such as herbs.’

Plant growth can be influenced by the colour of LED lighting. Marcelis refers to a test conducted on tomatoes in the Wageningen UR test greenhouses incorporating varicoloured LED lighting. Conventional lighting with red and blue light resulted in plants at chest height, while the plants in the test area that were exposed to far-red lighting grew above Marcelis’ head.

‘The research we are conducting should teach us which light combinations will result in optimum production. We are, for instance, also examining the results of applying far-red lighting for short periods during the night. Of all the spectral colours, red is the most efficient. Our knowledge of plant response to LED lighting is, however, still in its infancy.’

The research is funded by the STW technology foundation, LED lamp manufacturer Philips, three seed producing firms (Rijk Zwaan, Nunhems and Bejo), two automation firms (HortiMax and B-Mex), two plant nurseries (Van der Lugt and Westlandse Plantenkwekerij) and Wageningen UR University and Research Centre.

Leo F.M. Marcelis (Elst Gld, 1963) studied horticulture at Wageningen University, where he obtained his PhD in 1994. He was a professor by special appointment of Crop Production in Low-Energy Greenhouses at Wageningen University until 2013 and team leader at Wageningen UR Greenhouse Horticulture. On 1 December 2013 Prof. Dr Leo Marcelis was appointed Professor of Horticulture and Product Physiology at Wageningen University.